Apparatuses for achieving high pressures have been known for over a half century. Typical ultrahigh pressure apparatuses include piston-cylinder presses, cubic presses, tetrahedral presses, belt presses, girdle presses, and the like. Several of these apparatuses are capable of achieving ultrahigh pressures from about 4 GPa to about 7 GPa.
High pressure apparatuses are commonly used to synthesize diamond and cubic boron nitride (cBN), commonly known as superabrasives. In 2003, high pressure apparatuses provided a worldwide production of diamond superabrasives of about 600 tons, while about 200 tons of cBN superabrasives were produced. Generally, raw materials can be formed into a high pressure assembly and then placed in the high pressure apparatus. Under high pressure and typically high temperature, the raw materials form the desired product. More specifically, graphite or diamond can be used as a raw material in diamond synthesis, while hexagonal boron nitride (hBN) can be used in cBN synthesis. The raw material can then be mixed or contacted with a catalyst material. Diamond synthesis catalysts such as Fe, Ni, Co, and alloys thereof are commonly used. Alkalis, alkali earth metals, or compounds of these materials can be used as the catalyst material in cBN synthesis. The raw materials and catalyst material can then be placed in a high pressure apparatus wherein the pressure is raised to an ultrahigh pressure, e.g., 5.5 GPa. An electrical current can then be used to heat the catalyst material sufficient to melt the catalyst material, e.g., typically about 1300° C. Under such conditions, the raw material can dissolve into the catalyst and then precipitate out in a crystalline form as either diamond or cBN.
Unfortunately, currently known high pressure apparatuses and associated methods have expensive parts with limited useful life and limited available reaction volumes. For example, a typical belt apparatus includes an inner die which is shaped like a belt or doughnut, and concentric metal rings formed around the inner die as support. Early examples of belt apparatuses are described in U.S. Pat. Nos. 2,947,610 and 3,031,269, which are incorporated herein by reference. A pair of anvils is shaped to fit in the ends of the die opening. As such, the primary compression source is the pair of anvils which essentially shorten the length of the reaction volume and thus increase the pressure on the material placed therein. Because of the use of a die, the belt-type apparatuses may achieve ultrahigh pressures in a relatively larger reaction volume than typical cubic and tetrahedral presses which utilize retractable anvils without a die. Unfortunately, the die is typically formed of cemented tungsten carbide and concentric metal rings which are extremely difficult to make and involve considerable expense. Specifically, the die and concentric rings are assembled with highly precise interference fittings. Further, it is difficult to sinter a large die with high uniformity which can often result in localized areas which are structurally weaker. In addition, the die material is typically metal carbide, e.g., tungsten carbide, which has a very high compressive strength but relatively low tensile strength. As a result, these expensive dies frequently crack and fail due to extremely high hoop tension that develops around the circumference of the die as the die and concentric rings expand during advance of the pair of anvils.
Other methods for achieving ultrahigh pressures include cubic and tetrahedral presses which utilize multiple advancing anvils to press a sample. One such device is described in U.S. Pat. No. 3,159,876, which is incorporated herein by reference. Cubic presses and belt-type apparatuses can be used for diamond synthesis. However, the reaction volumes of cubic presses are somewhat smaller than belt-type apparatuses.
Therefore, apparatuses and methods which overcome the above difficulties would be a significant advancement in the area of high pressure devices.